1. Field of the Invention
The present invention relates to a data transmission cable and the like having a structure suitable for digital transmission.
2. Related Background Art
As a data transmission cable, a differential data transmission cable, for example, comprises a structure in which a shield is provided so as to cover a pair of conductors each coated with an insulator. Since the shield itself cannot be an ideal conductor, an eddy current occurs when an electric field is formed on the shield. It has been known that apparent conductor resistance increases due to the Joule loss caused by the occurrence of the eddy current thus confined within the shield.
Conventionally, for reducing such a Joule loss, it has been necessary to lower the ohmic value of the shield, whereby measures, for example, such as using a metal film with a high conductivity as a shield or preparing a shield having a sufficient thickness, for example, have been taken.
The inventors studied conventional data transmission cables and, as a result, have found a problem as follows. Namely, due to skin effect, the eddy current generated within a shield is distributed closer to the surface as the frequency is higher, while having the same frequency as that of signals transmitted. Therefore, the Joule loss becomes greater as the frequency is higher, whereby conductor resistance (Q/m) becomes greater as the frequency is higher as indicated by curve G100 in FIG. 1. In particular, the effectiveness of the thicker shield decreases as the frequency band for signal transmission is higher.
In the digital transmission using a conventional data transmission cable, there has usually been a problem that, due to the dependence of conductor resistance on frequency (curve G100 in FIG. 1), signal deterioration is caused by the increase of conductor resistance, which makes it difficult to maintain a sufficient transmission quality on the higher frequency band side.
In order to overcome the problem mentioned above, it is an object of the present invention to provide a data transmission cable comprising a structure for suppressing signal distortions by improving the frequency dependence of cable attenuation in digital transmission; and a communication method, a system, and a cord equipped with a connector which utilize the data transmission cable.
For achieving the above-mentioned object, the data transmission cable according to the present invention is directed to a differential data transmission cable which can yield an excellent effect of reducing the frequency dependence preferably in a transmission band of 100 Mbps to 3 Gbps. It comprises at least a pair of conductors, each coated with an insulator, extending along a predetermined direction; and a shield tape, disposed so as to surround the insulated conductors, including a metal layer covering the insulated conductors. In particular, in the shield tape, the metal layer covering the insulated conductors has a thickness of 1 xcexcm or more but 10 xcexcm or less, preferably 2 xcexcm or more but 6 xcexcm or less.
Here, a skin thickness which is the depth into the shield tape of distribution of an eddy current generated on the shield tape accompanying digital transmission, as the thickness of the metal layer, is given by the following expression (1):                     1                              π            ·            δ            ·            μ            ·            f                                              (        1        )            
where f is the fundamental frequency (Hz) of digital signals transmitted, xcex4 is the conductivity (mho/m) of the metal layer, and xcexc is the magnetic permeability (H/m) of the metal layer.
Here, with respect to the digital signal transmitted, the thickness of the metal layer is designed so as to become 50% or more but 300% or less of the skin thickness given by the above-mentioned expression (1).
The data transmission cable comprising a shield tape including the above-mentioned metal layer can reduce the eddy current confined within the metal layer but cannot at all prevent the eddy current from being generated. Therefore, the present invention controls the shield tape, the thickness of the metal layer in particular, so as to intentionally enhance and reduce the conductor resistance on the lower and higher frequency band sides, respectively, as indicated by arrows A1 and A2 in FIG. 1, thereby realizing a reduction in the frequency dependence of cable attenuation over the whole signal wavelength band, i.e., gain flattening. The data transmission cable according to the present invention uses a technique in which the conductor resistance generated by the eddy current confined within the metal layer included in the shield tape is positively utilized on the lower frequency band side in particular, so that no signals are required to be transmitted directly, whereby similar effects can be obtained whether the metal layer is grounded or not. The transmission band used for the data transmission cable according to the present invention includes at least one of the lower frequency band, in which the conductor resistance is enhanced, and the higher frequency band, in which the conductor resistance is reduced. Namely, the scope of the present invention includes a structure and usage for reducing the frequency dependency.
The shield tape may be constituted either by the metal layer alone or by a multilayer structure composed of the metal layer and an insulating layer such as a plastic material. When the shield tape comprises a multilayer structure, the metal layer is arranged so as to cover the insulated conductors.
The data transmission cable according to the present invention may comprise a drain wire extending in the predetermined direction while in a state accommodated inside the shield tape together with the insulated conductors. Also, the data transmission cable may comprise an outermost layer of an insulating material arranged on the outer periphery of the shield tape. Inversely, when the shield tape comprises a multiplayer structure and the data transmission cable does not have the drain wire, the metal layer can be arranged on the outer periphery of the shield tape.
The data transmission cable according to the present invention may comprise a metal material layer disposed so as to surround the outer periphery of the shield tape. When an outermost layer is provided so as to surround the outer periphery of the shield tape, it is preferred that the metal material layer be disposed between the shield tape and the outermost layer.
The data transmission cable according to the present invention may include a plurality of cable units each having a structure identical to that of the data transmission cable having the structure mentioned above.
A transmission system employing the data transmission cable comprising the above-mentioned structure realizes a communication method which effectively reduces the frequency dependence of cable attenuation preferably in a transmission band including a signal wavelength band (100 Mbps to 3 Gbps). When a cord equipped with a connector in which a connector is connected to a leading end of the data transmission cable is constructed, it can be applied to various systems such as semiconductor tester apparatus, LAN, high-speed computer line.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.